THE DESTRUCTION OF BACTERIA 91 



N 

 A 10 c.c. portion is rapidly titrated with boric acid solution (31 grams 



2i 



per liter), using powdered phenolplithalein as indicator (the usual alcoholic 

 solution of phenolphthalein will not serve) in order to determine the amount 

 of boric acid to be added to the rest of the filtrate. The end-point is the 



N 

 disappearance of the pink color. Each cubic centimeter of - - boric acid 



2i 



required for the 10 c.c. sample calls for the addition of 3 grains boric acid 

 per liter of filtrate. An excess of boric acid should be avoided, as it favors 

 the liberation of hypochlorous acid and renders the solution less stable. It 

 is best to add slightly less than the calculated amount. The concentrated 

 solution thus prepared contains about 4 per cent of sodium hypochlorite, 

 and before use should be diluted with about 7 parts of water and titrated 



N 

 with thiosulphate to determine its precise hypochlorite concentration. It 



10 



is then accurately diluted to the required strength (usually 0.5 to 0.45 per 

 cent). 



Preparation from Chlorine and Sodium Carbonate. Chlorine may be ob- 

 tained as the compressed gas in steel cylinders and is easily measured by 

 a chlorine meter manufactured for the purpose. This is a stable, economical 

 and convenient source of chlorine. A solution is prepared containing 15 

 grams of dry sodium .carbonate per liter (=17.6 grams monohydrate or 

 40 grams of washing soda). A measured quantity, 4.8 grams (or about 

 1600 c.c.) of chlorine gas is allowed to run into the solution for each liter. 

 Ten c.c. of the solution is then titrated. If the solution is too weak more 

 chlorine is introduced. If the solution is too strong it should be diluted 

 to a concentration of 0.5 per cent NaOCl with 1.5 per cent sodium carbonate 

 solution, which at the same time serves to correct the unduly diminished 

 alkalinity caused by the excess of chlorin introduced into the solution. If 

 the final solution fails to give a momentary flash of color with alcoholic 

 solution of phenolphthalein, it should be rejected. If the solution shows 

 color with powdered phenolphthalein, it must be titrated with boric acid as 

 described above, for preparation from bleaching powder, until this defect 

 is corrected, or it must be discarded. The solution should be titrated for 

 hypochlorite concentration every twenty-four to forty-eight hours and dis- 

 carded when it falls below the desired lower limit (usually 0.45 per cent). 



If a chlorin meter is not available, chlorin gas may be run into the 

 1.5 per cent carbonate solution through any improvised diffuser until the 

 hypochlorite concentration has reached 0.5 per cent. The amount of chlorin 

 required to give a hypochlorite concentration of 0.5 per cent is* approximately 

 twice the amount required to cause decolorization of powdered phenol- 

 phthalein. It is therefore convenient to add powdered phenolphthalein and 

 note the amount of chlorin required to cause its decolorization. When 



